In certain motor vehicle applications, a friction starting clutch is connected in the vehicle driveline between the engine and the drive wheels such that disengagement of the clutch effectively unloads the engine when the vehicle is at rest. To accelerate the vehicle from rest, the clutch is engaged at a controlled rate to progressively increase its torque capacity. As a result, the clutch transmits a controlled amount of the engine output torque to the drive wheels, and the vehicle is smoothly accelerated.
Generally, the starting clutch is engaged by fluid pressure, necessitating the use of an electrical-to-hydraulic actuator if engagement of the clutch is electronically controlled. When the actuator takes the form of a solenoid-operated fluid valve, the engagement pressure, and hence the torque capacity of the clutch, is controlled by energizing the solenoid at a variable duty cycle, Controllers for clutches of this type generally operate in open loop fashion. That is, the controller arrives at an energization command for the actuator and the resulting engagement pressure is not directly measured. Rather, the controller relies on a known relationship between the energization duty cycle and the resulting torque capacity of the clutch. Unfortunately, however, this relationship tends to vary--not only from actuator to actuator but with other constraints such as temperature and voltage. In addition, the performance of the clutch may vary with age and wear. As a result, the engine speed response during engagement of the clutch is not repeatable and the vehicle performance or efficiency may become impaired.